Many types of electronic test equipment (e.g., oscilloscopes) often involve the probing of a circuit of interest with a hand held probe. The probe might acquire a single-ended signal or a differential one, and there may or may not be a ground connection using a “flying lead” (a short length of flexible insulated wire with an alligator clip or other fastener at the free end). In the old days when components were large and frequency response was limited, the ground was generally a flying lead originating at the probe and was connected someplace convenient, while the tip of the probe was either pressed against or hooked onto a lead of a component in the circuit whose behavior was being measured. While those circumstances still sometimes occur, today's high density packaging of fast or high frequency circuitry often precludes it, and users of test equipment have complained to the designers of that test equipment that the techniques for probing need improvement.
In particular, it is often necessary to probe the signals at two places on a PCB (Printed Circuit Board) that: (1) Are some arbitrary distance apart; (2) Are traces leading to surface mounted components with no leads around which a probe tip may be hooked, requiring that sharp probe tips be pressed into those traces; and (3) Carry signal that have high frequency components (say, in the Giga Hertz region).
To accomplish these tasks a number of desirable properties of such a probe have been identified, and various designs have been offered. These desirable properties include adjustable spacing between a pair of small sharp probe tips with spring loading. They are small to cooperate with high frequency operation. They are sharp to allow them to penetrate any protective coatings and stay in place by slightly gouging into the trace. At least one is spring loaded to help them stay in place and not slip, even though the operator's hand may move or wiggle slightly during the measurement.
The “smallness” mentioned in the preceding paragraph is actually a manifestation of several factors. First, size matters in high frequency circuitry; dimensions that exceed a significant fraction of a wavelength, even for a simple conductive path, are apt to add unwanted reactance that interferes with frequency response. Component densities nowadays are high, compared to the past, with more components crowded into smaller space. Those familiar with modem probes will appreciate that high frequency probes have been getting smaller, and that the reduction in size, necessary as it is, complicates their use. The reason for this is that it is difficult to provide small yet robust mechanical structures that incorporate adjustability and compliance while also observing good high frequency practices. Mechanical arrangements that require two hands to set probe spacing are a liability, as there often simply isn't room to hold the probe in place with one hand while moving an adjustment lever with a finger of the other hand. We need a small adjustable and complaint high frequency probe that can be easily used and adjusted with just one hand. In fact, given the foreseeable bandwidth (10 to 12 GHz), we need it to be just as small as we can manage. Such a small probe is (excuse the pun) quite a large order. What to do?